Not Applicable.
This invention relates to scroll compressors, and in particular to a two-stage vacuum pump having an improved pressure ratio and performance.
Scroll devices have been used as compressors and vacuum pumps for many years. In general, they have been limited to a single stage due to the complexity of two or more stages.
Oil free scroll compressors have many leakage points. Typically, the scrolls operate with a small running clearance between the scrolls. This is a leakage point and reduces performance of the compressor. There are also leakage points under the tip seal and at the blow hole on either side of the tip seal.
Briefly stated, a new and improved scroll vacuum pump includes a first stage including a first stage fixed plate having a spiral involute, a second stage including a second stage fixed plate having a spiral involute wrap, and an orbiting plate having upper and lower surfaces. An involute spiral extends from the orbiting plate bottom surface to define a first stage orbiting plate and a second involute spiral extends from the orbiting plate top surface to define a second stage orbiting plate. Preferably, the first stage involute wraps are taller than the second stage involute wraps.
The first and second stages each have inlets and outlets. The first stage inlet and second stage outlet are located at the approximate centers of the first and second stage fixed plates, respectively. The first stage outlet and second stage inlet are both at the periphery of the first and second stages, and are in fluid communication with each other so that gas will flow from the first stage to the second stage.
The fixed and orbiting involute wraps of the first and second stages mesh with each other to define chambers. The first stage chambers increase in size from the first stage inlet to the first stage outlet, and the second stage chambers decrease in size from the second stage inlet to the second stage outlet. Thus, the first stage expands the gas and the second stage compresses the gas. The first stage has an expansion ratio greater than or equal to one and the second stage has a compression ratio greater than or equal to one. The first stage expansion ratio can be the same as, or less than, than the second stage compression ratio.
The first and second stage fixed plates define a lower housing. An upper housing is mounted to the second stage fixed plate and has an end surface spaced from the second stage fixed plate defining an exit chamber into which the second stage exit ports. The upper housing includes an outlet from said exit chamber.
The scroll vacuum pump includes a motor mounted to an outer surface of the upper housing to drive the orbiting plate. The motor includes an output shaft which extends through the exit chamber and the second stage fixed plate. The orbiting plate is operatively connected to said output shaft to be driven thereby. The motor output shaft is at atmospheric pressure. Thus, no special sealing of the output shaft is required.
To increase the efficiency of the pump, the involute spirals are extended as close as possible to the center of the involute to delay porting of gas by 180xc2x0 of rotation.